Characterisation and source apportionment of air pollutants

The air quality in urban areas is of great concern because of increasing emissions from motor vehicles. Pollutants such as fine aerosol particle (PM2.5, PM1.0, etc.), containing significant fraction of metals and organic compounds, can penetrate into lungs and, therefore, are more likely to increase respiratory and mutagenic diseases. Furthermore, these particles also play a significant role in global climate change and can be transported over long distances by prevailing winds. In order to develop strategies for the prevention, control and abatement of air pollutants improved knowledge is required on the nature, source and extent of pollution.

Together with our partner organisations, our group is investigating the sources and chemical composition of particulate matter (PM10 and PM2.5). We also focus on source apportionment of non-exhaust pollutants using tunnel studies. These details are provided below:

Characterisation of air pollutants

Methods for characterising non-exhaust particles, chemical characterisation of SPM, PM10, PM2.5, concentration and trends of atmospheric PAHs, Platinum Group Elements in the Environment.

Methods for characterising Non-Exhaust Particles

Samples of PM10 collected within the Hatfield Tunnel, UK during sampling campaigns are being analysed for metal and organic concentrations using ICP-AES and GC/MS respectively. Work carried out during the first sampling campaign illustrated the feasibility of the overall methodology of chemically analysis of species associated with sampling particulate matter and the use of receptor modelling to apportion the sources was proven. Development of the sample digestion processes was undertaken to further improve the methodology to achieve higher analytical sensitivities for a number of species.

An investigation was carried out looking at how the digestion of atmospheric particle samples could be improved to determine metal concentrations. The examination of different acid matrices such as hydrofluoric acid along with different digestion processes such as microwave digestion has been undertaken. This has led to the development of a sampling method using Partisol samplers incorporating Teflon filters, which could be employed alongside HiVol samplers. The additional Teflon filters samples will improve the detection of elements such as Al, Ca, Mg, Ba and Na which are associated with resuspended road dust and brake wear, and make a large contribution to urban particulate matter.

As the samples are also analysed for PAHs and Benzothiazole, work has been undertaken to develop a method to extract these compounds from the samples. The combined organic and metal data will help explain a large proportion of the particle mass, which in turn will improve the results of the source apportionment study. This will assist the investigation of non-exhaust emissions within the PM10 size fraction and unable the quantification of their emissions within the urban atmosphere.

Concentration and Trends of Atmospheric PAHs

There is an increasing concern about the occurrence of polycyclic aromatic hydrocarbons (PAHs) in the environment as they are ubiquitous in ambient air and some of them are among the strongest known carcinogens. PAHs and their derivatives are produced by the incomplete combustion of organic material arising, partly, from natural combustion such as forest and volcanic eruption, but with the majority due to anthropogenic emissions. Many countries have proposed a non-mandatory concentration limit for PAHs, whereas the health risk studies conducted in relation to PAH exposure, urge that these pollutants should be given a high priority when considering air quality management and reduction of impacts. Our team members are putting their efforts into evaluating the distribution and concentration of trends of PAHs. This will provide information for health risk studies and help in the comparison of modelled and measured values.

Platinum Group Elements in the Environment

Accumulation of platinum group elements (PGEs) is increasing in the environment over the time. Catalytic converters of modern vehicles are considered to be the main sources of PGE contamination since the Pt:Rh ratio of around 5 in various environmental compartments reflects the ratio in converter units (Ravindra et al., 2004). This study also shows that the concentrations of these metals have increased significantly in the last decades in diverse environmental matrices like airborne particulate matter, soil, roadside dust and vegetation, river, coastal and oceanic environment.

It is still under discussion whether the emitted PGMs are toxic for living organisms, and human beings. It is known that the metallic form of these elements is inert as far as biological reactions are concerned, but in contrast, some of their compounds, such as hexacholoroplatinate, and tetrachloroplatinite complexes, etc., are among the most potent allergens and sensitizers. Hence, the potential health risk from these elements would have to be taken in consideration especially in terms of exposure for those living in urban environments or along major highways. In this context, the transportation, transformation, and bioavailability of PGEs play a key role, especially, in the view of some contradictory results of the relevant literature (eg. estimates on the soluble PGE portion of the evolved exhaust, and interpretation on the PGE uptake by plants). All the above topics need further investigation (both experimental and model), partly on the base of health studies of PGM salts, to reach a better understanding of the behaviour of PGEs in the environment.

Source apportionment

Investigation of Non-Exhaust Emissions using Tunnel Measurements

Information relating to particle concentrations from non-exhaust sources is mainly based on simulated driving tests which have a number of uncertainties associated with them. In order to assist with identifying non-exhaust sources and quantifying their emissions, their physical properties and chemical composition needs to be established. By using tunnel measurements to collect samples for use within source apportionment studies allows the characterisation of particles emissions from road transport including non-exhaust sources. Instrumented road tunnels act as large laboratories; they allow the characterisation of particulate matter from a mixed, on-road fleet. By sampling simultaneously the particles at the tunnel entrance and inside the tunnel, the difference in concentrations can be assigned to the traffic activity.

Samples of PM10 collected within the Hatfield Tunnel, UK are analysed for metal and organic compounds. This combined metal and organic dataset will produce a unique and possibly the first UK dataset of this type for source apportionment of non-exhaust sources. Through source apportionment studies it is possible to investigate the sources of atmospheric particles. Using this technique will allow the examination of non-exhaust emissions and gain a greater understanding of the mechanisms in which this particles are generated. This will assist to develop pollution control and abatement strategies.

Emission Factors and Source Attribution of PAHs

There is an increasing concern about the occurrence of polycyclic aromatic hydrocarbons (PAHs) in the environment as they are ubiquitous in ambient air and some of them are among the strongest known carcinogens. PAHs and their derivatives are produced by the incomplete combustion of organic material arising, partly, from natural combustion such as forest and volcanic eruption, but with the majority due to anthropogenic emissions. The PAH concentration varies significantly in various rural and urban environments and is mainly influenced by vehicular and domestic emissions. Our research work aims to identify and characterize the emission sources of PAHs using various approaches such diagnostic ratio (DR), hierarchical cluster analysis (HCA) and principal component analysis (PCA). These approaches allow individual PAHs to be associated with their origin sources. Furthermore, we also wish to investigate the factors that effect PAH emission and their reactivity in atmosphere. A major FP5 project, SAPPHIRE has undertaken source apportionment in several European cities.

Air pollutants on regional scales

Air mass backward trajectories provide a useful means of establishing source-receptor relationships of air pollutants. Pollutants emitted from various sources, can remain in the atmosphere sufficiently long to be transported thousands of kilometres and thus to spread over a large area, across national borders, far from the original sources of polluting emission. There are many studies, which confirm that the general atmospheric circulation leads to long-range transport of aerosol or suspended particulate matter over various regions of the world. These aerosols carry a complex mixture of various inorganic and organic species including PAHs, originating from different sources. As mentioned above, a number of studies investigate the origin and transboundary movement of inorganic constituents of aerosol but little attention has been given for organic constituents and specifically for PAHs.

The variations in PAHs levels may be evaluated with relation to various emission sources and backward air mass trajectories to study the regional / global impact. The understanding of organic pollutants behaviour across national borders provides an aid to manage regional/ global air pollution control strategies.

References:

Ravindra, K., Wauters, E., Van Grieken, R., 2008. Variation in particulate PAHs levels and their relation with the transboundary movement of air masses. (Revision submitted to Science of the Total Environment)